The invention relates generally to distillation rate analysis and more particularly to automatic distillation analyzers for liquid hydrocarbons.
Over the years petroleum engineers have developed standards for commercial grade petroleum products. These standards are used by refineries in quality control to adjust process variables so that the resulting product more closely meets the prescribed standards. Wholesale purchasers of petroleum products such as gasoline and No. 2 fuel oil require the fuels they buy to meet these standards and manufacturers who design and build equipment which burns or otherwise uses these fuels and solvents rely on these standards.
Distillation profiles yield one of the major distinguishing features of motor gasolines, aviation gasolines, aviation turbine fuels, special boiling point spirits, naphthas, benzene, toluene, diesel fuel, white spirits, kerosenes, gas oils, distillate fuel oils, and similar petroleum products. For example, when gasoline is heated even slightly, the collected vapor can be condensed in an ice bath. Unlike water, for example, gasoline contains a diversified population of hydrocarbon molecules with different boiling points. Thus, in order to boil away or distill an entire sample of gasoline, the heat must be continually adjusted upwards and if one were to sense the vapor temperature above the boiling gasoline, it would reveal a characteristic increasing profile with respect to the evaporated percentage of the sample.
The test method for this type of distillation analysis has been prescribed by the American Society for Testing Materials (ASTM). Test designation D86, for example, prescribes a technique of performing laboratory distillation tests on gasoline. According to this test procedure, for example, a standard 125 millileter (ml) distillation flask with a 100 ml charge of gasoline are held to a beginning temperature between 55.degree. and 65.degree. F. The condenser bath must be 32.degree. to 34.degree. F. and the graduated cylinder or "graduate" for receiving the distillate must be between 55.degree. and 65.degree. throughout the test. The time from the first application of heat to the initial boiling point, i.e., first drop of distillate, must be between 5 and 10 minutes. The time from the initial boiling point to 5% (5 ml) recovered must be between 60 and 75 seconds. The uniform average rate of condensation from 5% recovered to 5 ml of the residue left in the flask must be 4 to 5 ml per minute (ml/min) and the time from the 5 ml residue to the end point must be in the range of 3 to 5 minutes. Vapor temperatures are read and recorded at the initial boiling point 5% recovered, 10% recovered and every 10 ml to 90% recovered, then 95% and at the end point (max. temp.). The percent recovered and residue are measured and recorded and percent loss is computed. The percent recovered is corrected to the prescribed percent evaporated by subtracting the loss percentage from the percent recovered. The thermometer reading at the prescribed evaporated percentages must then be determined and corrected for barometric pressure to represent the comparable reading at 760 millimeters (mm) pressure.
Because of the precise distillation rates required, this delicate procedure requires highly skilled laboratory technicians. Instrumentation has been developed which automatically adjusts the distillation flask heater in an attempt to stay within the prescribed distillation rate range. A travelling optical meniscus detector has been used before in this kind of apparatus to follow the liquid level in the receiving graduate. XY plotting strip chart recorders have been used to automatically record vapor pressure as the meniscus detector rises. These charts in and of themselves, however, do not yield the prescribed data without the barometric residue, and loss computations. Consequently, the temperature data must be read from the chart and recorded elsewhere for the prescribed evaporated percentages. This cumbersome recording process also applies to the time intervals for initial boiling point, 5% recovered and time from 5 ml residue to end point.
Even the automated D86 test for gasoline takes well over a half hour for each 100 ml sample not including whatever time is required for recomputation and recording of the variable parameters. Many of these instruments are installed at refineries where a variety of petroleum products are continuously produced. Thus, the plant engineers eagerly await the test results which may indicate that a process variable must be adjusted. In the meantime, unsaleable product may be produced. Accordingly, test errors or test results which must be scrapped because of erroneous procedure or recording techniques are very serious. Since the distillation rate test and recording requirements are complex, operator errors in the past have been frequent. Operators, being human, misrecord the sample identification number, neglect t precool the temperature sensor, forget to monitor or record the time from the initial boiling point to the 5% recovered mark, allow the condenser bath temperature to deviate from the norm during the test or fail to properly correct for loss in barometric pressure, etc.
To compound these difficulties, in prior instrumentation means for making the temperature sensor readings duplicate those of a laboratory thermometer standard have been lacking in accuracy. Moreover, the meniscus detector mechanism has heretofore been less precise than desired.